1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same suitable for an image pickup optical system, for example, such as a digital still camera, a digital video camera, a TV camera and a security camera, a broadcast camera.
2. Description of the Related Art
A zoom lens which has a high zoom ratio and is capable of performing high speed and high accuracy autofocus is required for an image pickup optical system used for an image pickup apparatus.
A positive lead type zoom lens which includes a lens unit having a positive refractive power on the most object side is known as a zoom lens having a high zoom ratio. Additionally, a rear focus type zoom lens which focuses by moving lens units except for a lens unit arranged on the most object side is known as a zoom lens easily performing high speed autofocus.
Additionally, recently, a single lens reflex camera is required for having a video recording function and performing autofocus during recording a video. A contrast system evaluating focusing state of an image pickup optical system by detecting high frequency component in an image pickup signal is widely used as an autofocus system during recording a video.
An image pickup apparatus using a contrast system detects a displacement direction from focusing state by vibrating a focus lens unit in an optical axis direction at high speed (hereinafter referred to as “wobbling”). And after wobbling, an optimum position of the focus lens unit becoming focusing state is calculated by detecting signal component of a specific frequency band of an image region from an output signal of an image pickup sensor. Then, the focus lens unit is moved to the optimum position so as to complete focusing. It is desirable that the focus lens unit is small and light so as to operate the focus lens unit at wobbling.
It is good for zoom lens having a high zoom ratio to contrive miniaturization and weight reduction of the lens by reducing the number of the lenses so as to perform high speed autofocus. However, when the number of the focus lens unit is easily reduced, aberration variation generated by focusing is larger. Therefore, it is important to properly select a zoom type and a focus lens unit in order to achieve high speed focus easily, make aberration variation smaller when focusing and acquire high optical performance in general object distance.
It is important to properly set transfer condition according to zooming of a plurality of lens units so as to acquire a high zoom ratio and miniaturization of an entire system simultaneously. Additionally, it is important to properly set imaging magnification of the focus lens unit and a refractive power arrangement of a lens unit at an object side and a lens unit at an image side in the focus lens unit so as to make aberration variation according to focusing smaller and acquire high optical performance in general object distance.
U.S. Pat. No. 6,556,356 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, and moves the second lens unit during focusing.
In U.S. Pat. No. 6,556,356, a movement amount during focusing becomes smaller by performing focusing of the second lens unit having strong negative refractive power, but since the lens unit is comparatively large size, the focusing speed is prone to be slow and a wobbling operation corresponding to a video is hard.
Japanese Patent Laid-Open No. 2001-033697 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power and a fifth lens unit having a positive refractive power, and moves the fourth lens unit during focusing.
In Japanese Patent Laid-Open No. 2001-033697, achievement of a small and light focus lens unit is contrived by performing focusing of the fourth lens unit, but optical performance is prone to change during zooming and focusing.
U.S. Pat. No. 8,472,123 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power and a fifth lens unit having a positive refractive power, and moves the third lens unit during focusing.
U.S. Pat. No. 8,243,170 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a positive refractive power and a fifth lens unit having a positive refractive power, and moves the third lens unit during focusing.
In the focus lens unit comprising a small number of the lenses in U.S. Pat. Nos. 8,472,123 and 8,243,170, when the power of the focus lens unit is strong, residual aberration of the focus lens unit and aberration variation by the object distance is larger. When the refractive power of the focus lens unit is weak, the entire system is prone to grow in size since the movement amount is larger during focusing.
Japanese Patent Laid-Open No. 2004-317867 discloses a zoom lens which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, a fifth lens unit having a positive refractive power, a sixth lens having a negative refractive power and a seventh lens having a positive refractive power, and moves the sixth lens unit during focusing.
In Japanese Patent Laid-Open No. 2001-033697, the negative refractive power of the sixth lens unit comparatively arranged on the image side is stronger and an enough moving space for focusing is prepared so as to make the focus sensitivity larger. Therefore, it is difficult to become a refractive power arrangement of a retrofocus type at a wide angle and contrive a wide angle of view.